Heretofore, various analysis techniques for analyzing the swing action of a golf club have been proposed in order to help in golf club fitting, golf club product development, improving a golfer's swing action, and the like. In many cases, this type of golf swing analysis involves measuring the swing action with a measurement device such as an acceleration sensor, an angular velocity sensor or a camera, and deriving the behavior of the golf club that occurs during the swing action based on the measurement values.
In analyzing the swing action, the behavior of the golf club may be modeled with the dynamic model of a double pendulum. For example, the behavior of the club can be represented with a model that takes the golfer's shoulder and the grip of the club (or the wrist of the golfer holding the grip) as nodes, and takes the golfer's arm and the golf club as links. Also, Patent Literature 1 (JP 2014-73314A) discloses a double pendulum model that takes the middle of both shoulders of the golfer and the grip as nodes.
In Patent Literature 1, in order to model the swing action with a double pendulum model, an inertial sensor is attached the golfer's upper body and the movement of the upper body is measured, and an inertial sensor is also attached to the golf club and the movement of the golf club is measured. However, when the measurement device is thus attached to various sites to measure the movement of the various sites in order to analyze the swing action with a double pendulum model, the measurement device becomes large scale. Attaching the measurement device to the golfer's body, particularly as in Patent Literature 1, impedes the golfer's natural movement, possibly resulting in less accurate analysis.
Also, various fitting methods for assisting selection of golf clubs suited to a golfer have heretofore been proposed. A typical fitting method involves getting the golfer to take practice hits with the golf clubs, measuring the swing action during this time with a measurement apparatus, and analyzing the measurement values to thereby select an optimal golf club. The criteria on which fitting is based at this time is an important factor in determining the quality of the fitting, with various criteria having been proposed to date. For example, Patent Literature 2 (JP 2013-226375A) discloses a fitting method that is based on the stiffness of the shaft of the golf club.